1. Field of the Invention
The present invention relates to semiconductor processing technology and, in particular, concerns a process whereby oxide structures can be grown to have finely calibrated thicknesses which has particular application in deep ultraviolet phase shift masks used in photolithography.
2. Description of the Related Art
Over the past 20 years, the semiconductor industry has seen a tremendous increase in the scale of integration of integrated circuits formed using semiconductor processing techniques. As the scale of integration has increased, so has the need for processing techniques that allow for the formation of devices and structures of increasingly smaller and smaller sizes. Moreover, there has also been an increased need for process techniques that will allow for the formation of structures on a substrate that are positioned closer and closer to each other.
The primary technique by which structures are formed on a substrate in semiconductor processing is known as photolithography. Generally, a photosensitive agent, known as a photoresist, is positioned on an upper surface of a substrate and a mask is positioned over the photoresist. The mask is typically patterned with opaque regions and transmitting regions so that light can be selectively provided to regions of the photoresist positioned on top of the wafer. The photoresist is light sensitive such that the regions of the photoresist receiving the light become exposed. Exposure of the photoresist chemically changes the material in this region thereby allowing for selective removal of the photoresist. Once the photoresist has been removed, structures can be formed on the substrate or within the regions of the substrate corresponding to the removed areas of the photoresist.
As device dimensions have significantly decreased and the scale of integration has significantly increased, there has been an ongoing need for lithography techniques that allow for the exposure of smaller and smaller regions of the photoresist and also regions of the photoresist that are positioned very close to one another.
One particular problem that occurs with masks that arc projecting light onto regions of photoresist that are very closely positioned together is that it can be difficult to selectively expose well-defined regions of the photoresist to the light without interference from light from adjacent openings in the mask. Moreover, as the openings in the mask are positioned closer and closer to each other, diffraction of the light that is travelling through the adjacent openings in the mask can result in the area between the two regions of the photoresist material on the substrate that are to be exposed also being exposed by the diffracting light.
To address this particular problem, prior art photolithography masks have been developed wherein the light that is traveling through one opening in the mask also travels through an additional layer of light transmitting material such that the light emanating therefrom is 180 degrees out of phase from the light traveling through the adjacent mask opening. The combination of diffracted light emanating from adjacent openings in the phase shift mask results in the light traveling to the area between the two regions of the photoresist material that are to be exposed destructively interfering such that the photoresist in this area is not exposed. Moreover, phase shifting the light also reduces the effect of light traveling through one mask opening affecting the exposure of the photoresist in the region corresponding to the adjacent mask opening. Hence, phase shift masks can be used to produce more highly defined integrated circuits with semiconductor structures placed much closer together thereby permitting increasing the scale of integration.
While phase shift photolithography has enhanced the resolution and depth of image of lithographic techniques, it underscores the need for the ability to produce very thin structures having a very highly calibrated thickness of material. As discussed above, there is an on-going need for the ability to form particular structures having very high thickness tolerances in all phases of semiconductor processing. In the particular application of phase shift photolithography, it is desirable to be able to produce phase shifting light transparent structures having a specific thickness so that the desired degree of phase shift of the light traveling through the mask opening can be obtained in order to achieve desired destructive interference between the light traveling through adjacent openings in the mask. The phase shift of the light is, of course, dependent upon the material through which the light is traveling and also the thickness of this material. However, using prior art techniques for forming structures of a light transparent material, such as silicon oxide, it is difficult to form very small structures with a desired degree of tolerance for the thickness of the structure. For example, using deep UV phase shift microlithography, it is desirable to be able to grow phase shift structures having a tolerance within approximately 12 Angstroms of the wavelength of the deep UV light (e.g., approximately 400-1200 Angstroms). Hence, there is a need for an ability to form structures, and in particular light transmitting structures, to within very high tolerances.